Optoelectronic device including digital filters compensating for component stipulated signal distortion in a recieved signal for recognizing barcode symbols

ABSTRACT

An optoelectronic apparatus is provided for identifying marks comprised of defined contrast patterns, particularly barcode symbols. The device includes a transmitting element which emits transmitted light and a receiving element. The transmitted light is guided across the marks, and the received light reflected by the marks has an amplitude modulation imposed by the contrast of the marks, with the received light being converted into a voltage signal that forms the received signal in the receiving element. The voltage signal is converted into a binary signal and filtered by an arrangement of digital filters for compensation component stipulated distortions prior to being fed to a threshold-value unit for evaluation of the contrast pattern.

BACKGROUND OF THE INVENTION

The invention relates to a method and an apparatus for compensatingcomponent-stipulated signal distortions in an optoelectronic apparatusfor identifying marks provided with defined contrast patterns, such asbar code symbols. Such an apparatus includes a transmitting element thatemits transmitted light, and a receiving element. The transmitted lightis guided across the marks and the received light reflected by the markshas an amplitude modulation imposed by the contrast of the marks,wherein the received light is converted in the receiving element into avoltage signal that corresponds to the received signal. The voltagesignal is fed to a threshold-value unit for evaluation of the contrastpatterns.

An apparatus of this type, which is used to read bar code symbols, isknown from JP4-346184A. Such apparatuses can preferably be configured ashand-held reading devices that are guided across the bar code symbols tobe read at relatively short distances.

For this purpose, the transmitting element includes, in addition to atransmitter preferably configured as a laser, a scanner thatperiodically diverts the transmitted light and guides it multiple timesacross the bar code symbols to be read.

In this type of application, it is sufficient that the transmitted lightbeam is guided across the bar code at a relatively low scanning rate.Typical scanning rates lie within a range of approximately 30-50scans/second.

The received light reflected by the bar code symbols has an amplitudemodulation that corresponds to the light-dark transitions of the barcode symbols. The signal frequencies are a function of the bar codepattern, on the one hand, and the scanning rate and the reading distanceon the other.

The received light is converted into a voltage signal and amplified inthe receiving element, which is typically formed by a photosensor,preferably a photodiode, and an amplifier.

The analog received signals are evaluated in the threshold-value unit.The voltage signals are preferably evaluated with a switching threshold,because of which the light zones of the bar code symbols, to which ahigh intensity of the received signal corresponds, can be distinguishedfrom the dark zones of the bar code symbols, to which a low intensity ofthe received signal corresponds.

Because the scanning rates at which the bar code symbols are scanned arerelatively low, and the reading distances are relatively small and areonly subjected to low fluctuations, the bar code symbols can be resolvedfrom the transmitted light beam of the apparatus, i.e., the differencein intensity between the received signals reflected by the light anddark surfaces, respectively, is considerably greater than the signaldistortions that take place in the components of the receiving elements.In this respect, reliable detection of the bar code symbols is assuredwithout measures imposed by the manufacturer to compensatecomponent-stipulated signal measurements.

An apparatus for identifying bar code symbols whose receiving element isformed by a line camera comprising a linear arrangement of photodiodesis known from U.S. Pat. No. 4,323,772.

The analog received signal registered by the line camera is digitized bymeans of a threshold-value unit. A digital filter is disposed downstreamof this threshold-value unit. An evaluation of the signals is performedwith the digital filter in that a certain state, i.e., a black or whiteline element of a bar code symbol, is only considered identified when atleast two adjacent photodiodes deliver the same received signal. In thisway, malfunctions of individual photodiodes can be compensated, or minorerrors in the bar code symbols can be suppressed.

However, in many cases, in the use of optoelectronic apparatuses of thetype mentioned at the outset in industrial settings, higher requirementsare placed on the resolution capability of the apparatus. The distanceof the apparatus from the bar code symbols can lie within a range of upto a few meters, and can possibly vary dramatically.

For example, bar code symbols can be applied to packages of differentsizes that are transported on a conveyor belt. The optoelectronicapparatus is preferably disposed at a fixed distance above the conveyorbelt. Depending on the speed of the conveyor belt, the reading distanceand size of the bar code symbols, scanning rates of the apparatus whichlie within a range of 1000 scans/second can be necessary. Typicalscanning rates lie Within a range of 300-1000 scans/second. At scanningrates of this order of magnitude, the received signal frequencies liewithin a range of 0.5 Mhz and higher.

In these types of applications, the signal adulterations causedprimarily by the components of the receiving element are of the sameorder of magnitude as the differences between the received signalsreflected by the light and dark surfaces, respectively, of the bar codesymbols. A possible result of this is that the bar code symbols can nolonger be decoded error-free by the optoelectronic apparatus.

SUMMARY OF THE INVENTION

An object of the invention is to configure an optoelectronic apparatusof the type mentioned at the outset such that reliable detection ofmarks provided with defined contrast patterns is assured at higherscanning rates.

The above and other objects are accomplished according to the inventionby the provision of a method of compensating component-stipulated signaldistortions for an optoelectronic apparatus for identifying marksprovided with defined contrast patterns, comprising: experimentallydetermining a transmission function of the signal-distorting componentsin their entirety; optically scanning the marks and converting, in thesignal-distorting components, received light reflected by the marks intoan analog, electrical received signal; converting the analog receivedsignal into a digitized received signal; filtering the digitizedreceived signal in an arrangement of digital filters having atransmission function selected such that the transmission functions ofthe signal-distorting components are linked with the transmissionfunction of the arrangement of digital filters within a predeterminablefrequency range to result in an essentially frequency-independenttransmission characteristic of a group delay time of the received signaland a Gaussian transmission characteristic of an amplitude of thereceived signal; and evaluating the filtered, digitized received signalfor identifying contrast patterns indicated by the received signal.

According to another aspect of the invention there is provided anoptoelectronic apparatus for identifying marks provided with definedcontrast patterns, comprising: a transmitting element that emitstransmitted light; means for guiding the transmitted light across themarks; a receiving element for receiving light reflected by the markswhich has an amplitude modulation imposed by the contrast of the marksand converting the received light into a voltage signal; athreshold-value unit for evaluation of contrast patterns indicated bythe received signal; and a compensation device connected between thereceiving element and the threshold-value unit and comprising an n-bitanalog-digital converter having a word width n larger than one forconverting the voltage signal into a binary signal sequence and anarrangement of digital filters disposed upstream of the threshold-valueunit.

The basic concept of the invention is to systematically detect theinterferences of the received signal that are caused particularly by thereceiving element, and to compensate them by means of an arrangement ofdigital filters. For this purpose, an n-bit analog-digital converterthat converts the analog received signal into a digital signal isdisposed downstream of the receiving element. It is useful to select theresolution of the analog-digital conversion to be as high as possible,i.e., the word width n of the analog-digital converter should beselected to be as large as possible. Because of this, an informationloss is extensively prevented during conversion of the analog signalinto a digital signal.

The compensation of the distortions of the receiving signal is effectedby a suitable selection of the transmission function of the arrangementof digital filters, to which the digitized received signal is supplied.

Knowledge of the transmission function of the receiving element or,possibly, of further components that distort signals is necessary fordetermining the transmission function of the digital filter. Thetransmission function of the receiving element is advisably determinedexperimentally.

The transmission function of the arrangement of digital filters isselected such that the linking of the transmission functions of thesignal-distorting components and the digital filters results in atransmission characteristic for the phase of the received signal that isindependent of frequency, and a Gaussian transmission characteristic forthe amplitude of the received signal. Thus, phase errors and amplitudeerrors are compensated equally by the apparatus of the invention.

A significant advantage of the invention is that the compensation fordistortion takes place via digital filters which can be dimensioned veryprecisely in their transmission characteristic over a large frequencyrange. Because of this, signal distortions in a frequency range of up toa few MHz can be eliminated. This means that error-free detection of thebar code symbols is assured, even at scanning rates in the kHz range.

The signal distortions caused by the signal-distorting components can beeliminated completely and systematically by the apparatus according tothe invention. In particular, distortions of the received signalsthemselves can be eliminated when they are of the same order ofmagnitude as the useful signals.

The dimensioning of the digital filter is effected by way of a suitableselection of filter coefficients. These filter coefficients can be setusing a software program. Thus, component tolerances can be changed in atime-saving manner without a hardware expenditure. The quality of theapparatus can therefore be increased cost-effectively and monitored.

In a useful embodiment of the invention, a recursive IIR filter is usedto eliminate the phase errors. A non-recursive FIR filter isadvantageously used to eliminate the amplitude distortions of thereceived signal.

These filters have a defined number of freely-selectable coefficients.The desired transmission function can be modelled very precisely throughthe selection of these coefficients.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below by way of the drawings. Shown are in:

FIG. 1: a fundamental construction of the optoelectronic apparatus,

FIG. 2: a block diagram of the evaluating unit of the optoelectronicapparatus according to the invention,

FIG. 3: pulse diagram of signal evaluation in the threshold-value unitincluding;

a) representation of a bar code symbol

b) received signal at the input of the threshold-value unit

c) differentiated received signal

d) binary received signal sequence at the output of the threshold-valueunit,

FIG. 4a: a block diagram of an IIR filter,

FIG. 4b: a block diagram of an FIR filter,

FIG. 5: a graph showing limiting frequency of the received signal as afunction of the reading distance,

FIG. 6: a graph showing frequency dependency of the group delay time andthe amplitude of the received signal in the receiving element,

FIG. 7: a graph showing frequency dependency of the group delay time andthe amplitude of the received signal in the arrangement of digitalfilters,

FIG. 8: a graph showing received signal as a function of time,

(1) at the input of the receiving element,

(2) at the output of the receiving element,

(3) at the output of the phase compensator,

(4) at the output of the pulse shaper.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Shown in FIG. 1 is the fundamental structure of an optoelectronicapparatus 1 according to the invention for identifying marks providedwith defined contrast patterns. In principle, the marks can havearbitrary sequences and shapes of contiguous light-dark surfaces,preferably black-white surfaces. The invention is explained below forthe case that the marks are formed by bar code symbols 2. The bar codesymbols 2 essentially comprise a sequence of black and white lineelements 2a, b, respectively, of defined length and width.

The optoelectronic apparatus 1 essentially comprises a transmittingelement 3, a receiving element 4 and an evaluating unit 5. Thetransmitting element 3 comprises a transmitter 6, preferably a laserdiode, and transmitting optics 7 which is disposed downstream of thetransmitter 6 and focuses the transmitted light 8. The focussedtransmitted light 8 is deflected by a mirror 20 onto a diverting unit 9which, in the present example, is formed by a rotating polygonal mirrorwheel, and guided to the bar code symbol 2 to be detected. The axis ofrotation of the polygonal mirror wheel is perpendicular to theequatorial plane of the polygonal mirror wheel illustrated in FIG. 1.

The received light 10 reflected by the bar code symbol 2 is guided tothe receiving element 4 by way of the polygonal mirror wheel. Thereceiving element 4 comprises a photodiode 11, in which the receivedlight 10 is converted into an electrical received signal, and anamplifier 12 disposed downstream of the receiving element. To improvethe detection sensitivity, receiving optics 13 is disposed upstream ofthe receiving element 4.

The received signal present at the output of the receiving element 4 issupplied to the evaluating unit 5.

In optoelectronic apparatuses according to the prior art mentioned atthe outset, the evaluating unit 5 solely comprises a threshold-valueunit 14 (FIG. 2). This type of apparatus is illustrated in FIG. 2 with adashed arrow line between amplifier 12 and threshold-value unit 14. Inthis case the received signal is supplied as an analog signal to theevaluating unit 5.

The evaluating unit 5 of the apparatus 1 of the invention has an n-bitanalog-digital (A/D) converter 15 and an arrangement 16 of digitalfilters disposed downstream thereof, the filters being disposed upstreamof the threshold-value unit 14. This evaluating unit 5 is characterizedby solid arrow lines in FIG. 2.

The principle of the evaluation of the received signals can be seen fromFIG. 3. A bar code symbol 2 comprising a series of black and white lineelements 2a, b, respectively, is illustrated in FIG. 3a. If the edgeerrors of the bar code symbol 2 are imperceptibly small, i.e., thecontrasts between black and white surfaces are very sharp, and if thebeam diameter of the transmitted light 8 on the bar code symbol 2 issignificantly smaller than the smallest width of a line element 2a, b,the transmitted light 8 is modulated in amplitude by the reflection fromthe bar code symbol 2, as shown in FIG. 3b.

If no distortion or adulteration of the received signal takes place inthe receiving element 4, the curve shape shown in FIG. 3b corresponds tothe received signal present at the output of the receiving element 3.

The determination of the width of the individual line elements 2a, b ofthe bar code symbol 2 in the evaluating unit 5 is advisably effectedaccording to the turning point method.

In a first step, the received signal is differentiated (FIG. 3c).Subsequently, the extremes of the differentiated received signal thatcorrespond to the turning points of the received signals are determined.These turning points in turn define the transitions from a black to awhite line element or vice versa.

For determination of the turning points of the received signals, thedifferentiated received signal is converted, preferably with twoswitching thresholds S₁ and S₂ (FIG. 3c), into a binary signal (FIG.3d). The duration of the `0` and `1` states of the binary signalsequence is a measure for the width of the line elements 2a, b of thebar code symbol 2. The duration of the `0` and `1` states can bedetected simply by means of a clock-controlled counter.

Because of signal distortions in the receiving element 4 of theoptoelectronic apparatus 1, the turning points of the received signalsequence can be displaced so significantly that a reconstruction of theline pattern of the bar code symbol 2 from the received signal is nolonger possible.

The arrangement 16 of digital filters disposed upstream of thethreshold-value unit 14 and an analog-digital converter 15 disposedupstream of this arrangement 16 are provided in the evaluating unit 5 inorder to eliminate these signal distortions.

The n-bit analog-digital converter 15 has a word width in a range ofn=8-12. In the present embodiment, an 8-bit analog-digital converter 15is used. Because of this, the analog received signal can be convertedinto a digitized received signal with a high resolution.

The arrangement 16 of digital filters disposed downstream of theanalog-digital converter 15 comprises two filters connected in series.The first filter is a phase compensator 17 that is used to eliminate thephase distortions of the received signal, while the second digitalfilter represents a pulse shaper 18 that is used to eliminate theamplitude distortions of the received signal.

The phase compensator 17 is formed by a recursive IIR filter. Thefundamental construction of a two-stage IIR filter is illustrated inFIG. 4). The output value of the IIR filter y_(n) at a time n is afunction of the input value x_(m) at different times m=n, n-1, n-2, . .. , n-M; the time n-M is an earlier time with respect to the time n by Mdiscrete time stages. In addition, the output value y_(n) is a functionof the output value y_(m) at different, earlier times m=n-1, n-2, . . ., n-M: ##EQU1##

The weighting of the influence values y_(m) and x_(m) is effected withcoefficients a_(m) or b_(m), which are adjustable. The number M ofcoefficients determines the degree of the IIR filter. In the presentembodiment, a series connection of three IIR second-degree filters isused.

The variable z shown in FIG. 4a is the variable in the frequency rangethat is a conjugate for time variables n. The value z⁻¹ represents thelength of the delay between two linkage points, e.g. x_(n) and x_(n-1).The symbols x and Σ, respectively, characterize a multiplicative oradditive linkage.

The pulse shaper 18 is formed by a non-recursive FIR filter. Thefundamental construction of an FIR filter is illustrated in FIG. 4b. Theoutput value y_(n) of the FIR filter is a function of the input valuex_(m) (m=n, n-1, n-2, . . . , n-M) at different times. ##EQU2##

The weighting of the input values x_(m) is effected with adjustablecoefficients h_(m). The number M of coefficients h_(m) determines thedegree of the filter. In the present embodiment, an 18 degree FIR filteris used.

The digitized and filtered received signal is fed to the threshold-valueunit 14 and converted into a binary signal sequence there.

FIG. 5 illustrates the upper limiting frequency of the received signalsof an optoelectronic apparatus 1 for a predetermined scanning rate of480 scans/second as a function of the reading distance d. The limitingfrequency lies within a range of a few hundred kHz, even above 500 kHzfor reading distances d greater than 70 cm. At these types of highreceived signal frequencies, the interferences in the receiving element4 typically cause distortions of the received signals of the same orderof magnitude as the useful signals.

To eliminate these interferences, the frequency dependency of thetransmission function of the receiving element 4 is determined in afirst method step, i.e., the frequency response of the amplitude and thegroup delay time of the received signal is determined in the receivedsignal. The group delay time is defined as the differential d ψ/d f,that is, the differentiation of the phase of the received signalaccording to the frequency. The response behavior of the receivingelement 4 can be usefully effected by the feeding in of a predeterminedsignal and measurement of the phase and amplitude of the output signal.The step response of the receiving element is advantageously measuredand converted into the pulse response.

The behavior shown in FIG. 6 results for the present embodiment. Thecharacteristics of the receiving element 4 deviate greatly from theideal behavior, namely a frequency-independent amplitude and afrequency-independent group delay time.

The basic idea of the method according to the invention is to dispose anarrangement 16 of digital filters downstream of the receiving element 4,the transmission function of the filters being such that signaldistortions are eliminated by the receiving element 4, i.e., that thetransmission function of the entire system comprising the receivingelement 4 and the arrangement 16 of digital filters is such that thefrequency response of the group delay time is independent of frequency,at least in the frequency range of the received signals (FIG. 5), andthe frequency response of the amplitude has a Gaussian characteristic inthis frequency range.

This is achieved on the one hand in that the frequency response of thegroup delay time of the received signal in the phase compensator 17 isessentially formed in the receiving element 4 by the difference betweena constant and the frequency response of the group delay time of thereceived signal (FIG. 7). The coefficients of the digital filter formingthe pulse shaper 18, on the other hand, are selected such that they donot influence the group delay time of the received signal.

On the other hand, the coefficients of the pulse shaper 18 are selectedsuch that the product of the transmission functions of the receivingelement 4 and the pulse shaper 18 has a Gaussian characteristic; themaximum of the Gaussian transmission function lies at the frequency f=0,and at higher frequencies it drops off to a frequency f₁, at which thetransmission function assumes the value 0. In a useful manner, thecoefficients of the digital filter forming the phase compensator 17 areselected such that the phase compensator 17 alone influences the groupdelay time, but not the amplitude of the received signal.

Because of this, it is accomplished that solely the phase characteristicis optimized by means of the phase compensator 17, and solely theamplitude of the received signal is optimized by means of the pulseshaper 18.

Because the amplitude in the receiving element 4 is very low at highfrequencies, or assumes the value zero (FIG. 6), the amplitude in thepulse shaper 18 would have to become very large at high frequencies orassume the value infinity in order to assure an ideal transmissionbehavior.

Since this cannot be realized, a deviation from the ideal behavior isobtained above a limiting frequency f₀ for the series connection ofreceiving element 4 and pulse shaper 18, namely an amplitude thatdecreases with increasing frequency. The frequency response of the groupdelay time has the same behavior.

Provided that the limiting frequency f₀, at which the transmissionfunction of the entire system of receiving element 4, phase compensator17 and pulse shaper 18 uses the frequency dependency, lies above thelimiting frequency of the received signals, an essentiallydistortion-free transmission behavior is obtained. The coefficients ofthe IIR filter and the FIR filter are selected so that this condition ismet.

The influence of the digital filters on the signal course of thereceived signal is illustrated in FIG. 8. For the sake of an overview,the different curve shapes are shown staggered in time in FIG. 8.

The ideal, non-distorted received signal (1) comprises a step function.This corresponds to the transition from a black to a white line element2a, b of a bar code symbol 2.

As it passes through the receiving element 4, the received signal isdistorted and has the curve shape indicated by (2). The phase errors ofthe received signal (3) are extensively eliminated at the output of thephase compensator 17, so that the overshoots are configured symmetricalto the time axis when the received signal increases from the signalvalue of 0 to the signal value of 1. The received signal is smoothed bythe pulse shaper 18, so virtually no more overshoots are present in thesignal curve (4) at the output of the pulse shaper 19. As a result,through the use of the digital filters 17, 18, the ideal signal course(1) is nearly recovered from the received signal (2) distorted in thereceiving element.

The arrangement 16 of digital filters according to the invention assuresa compensation of component-stipulated signal distortions. Onlycomponent parameters, particularly the characteristic of thetransmission function of the receiving element 4, are considered in theselection of the coefficients of the filters 17, 18. Consequently, thiscompensation is not a function of the parameters of the transmittedlight beam 8, and particularly of the reading distance d, the beamdiameter and the beam shape.

We claim:
 1. A method of compensating component-stipulated signaldistortions for an optoelectronic apparatus for identifying marksprovided with defined contrast patterns, comprising:determining acomponent specific transmission function of the signal-distortingcomponents in their entirety; optically scanning the marks andconverting, in the signal-distorting components, received lightreflected by the marks into an analog, electrical received signal;converting the analog received signal into a digitized received signal;filtering the digitized received signal in an arrangement of digitalfilters having a transmission function selected such that the componentspecific transmission function of the signal-distorting components islinked with the transmission function of the arrangement of digitalfilters within a predeterminable frequency range to result in anessentially frequency-independent transmission characteristic of a groupdelay time of the received signal and a Gaussian transmissioncharacteristic of an amplitude of the received signal; and evaluatingthe filtered, digitized received signal for identifying the contrastpatterns.
 2. The method according to claim 1, wherein thepredeterminable frequency range encompasses the frequencies of thereceived signal.
 3. An optoelectronic apparatus for identifying marksprovided with defined contrast patterns, comprising:a transmittingelement that emits transmitted light; means for guiding the transmittedlight across the marks; a receiving element for receiving lightreflected by the marks which has an amplitude modulation imposed by thecontrast of the marks and converting the received light into a voltagesignal; a threshold-value unit for evaluation of contrast patternsindicated by the received signal; and a compensation device connectedbetween the receiving element and the threshold-value unit andcomprising an n-bit analog-digital converter having a word width nlarger than one for converting the voltage signal into a binary signalsequence and an arrangement of digital filters disposed upstream of thethreshold-value unit for compensating component-stipulated distortionsof the received signal.
 4. The apparatus according to claim 3, whereinthe arrangement of digital filters includes two digital filtersconnected in series.
 5. The apparatus according to claim 3, wherein afirst one of the digital filters comprises a phase compensator whichchanges the group delay time of the received signal in afrequency-selective manner.
 6. The apparatus according to claim 5,wherein the group delay time of the received signal in the phasecompensator has a frequency response that essentially corresponds to adifference between a constant and a frequency response of the groupdelay time of the received signals in the receiving element.
 7. Theapparatus according to claim 5, wherein the phase compensator comprisesan IIR filter having adjustable coefficients.
 8. The apparatus accordingto claim 5, wherein a second one of the filters comprises a pulse shaperwhich changes the amplitude of the received signal in afrequency-selective manner.
 9. The apparatus according to claim 8,wherein the frequency response of the amplitude of the received signalin the pulse shaper and the frequency response of the amplitude of thereceived signal in the receiving element have a product that essentiallycorresponds to a Gaussian function below a limiting frequency.
 10. Theapparatus according to claim 9, wherein the limiting frequency lieswithin a range of 1-2 MHz.
 11. The apparatus according to claim 8,wherein the pulse shaper comprises an FIR filter having adjustablecoefficients.
 12. The apparatus according to claim 11, wherein the FIRfilter comprises an 18 degree filter.
 13. The apparatus according toclaim 3, wherein the word width of the n-bit analog-digital converter iswithin a range of 8≦n≦12.
 14. The apparatus according to claim 3,wherein the transmitting element includes a transmitter comprising alaser and a diverting unit for guiding the transmitted light across themarks, and the receiving element comprises a photodiode and an amplifierconnected to the photodiode for amplifying the received signal.